1. Field of the Invention
This invention relates to a position controller for an electric motor such as a DC motor, an induction motor, or a synchronous motor, which drives a load machine such as a table of a machine tool, or an arm of an industrial electric robot.
2. Description of the Related Art
The configuration of a prior art example will be described with reference to a figure. FIG. 14 is a block diagram showing an example of a prior art position control system for an electric motor. In the figure, reference numeral 1 designates a rotation angle instruction signal generation circuit; 2, a position control circuit; 3, a speed control circuit; 10, a torque control circuit; 11, an electric motor; 12, a torque transmission mechanism; 13, a load machine; and 14, a rotation detector. The motor 11 drives the load machine 13 via the torque transmission mechanism 12. The rotation detector 14 detects the speed and the position of the motor 11 and outputs of a speed signal .omega..sub.M and a real rotation angle signal .theta..sub.M. The position control circuit 2 comprises a subtracter 20 which outputs a deviation (.theta..sub.M *-.theta..sub.M) between a rotation angle instruction signal .theta..sub.M * output from the rotation angle instruction signal generation circuit 1 and a real rotation angle signal .theta..sub.M, and a coefficient unit 21 which proportionally amplifies the deviation (.theta..sub.M *-.theta..sub.M) and outputs a speed instruction signal .omega..sub.M *, and conducts a control so that the signal .theta..sub.M * follows the signal .theta..sub.M. The speed control circuit 3 comprises a subtracter 22 which outputs a deviation (.omega..sub.M *-.omega..sub.M) between the speed instruction signal .omega..sub.M * and a real speed signal .omega..sub.M, and a proportional integrator 50 which proportion-integrally amplifies the deviation (.omega..sub.M *-.omega..sub.M) and outputs a torque instruction signal T.sub.M *, and conducts a control so that the signal .omega..sub.M follows the signal .omega..sub.M *. The torque control circuit 10 controls the torque of the motor 11 in accordance with the torque instruction signal T.sub.M *.
In the prior art position controller for an electric motor, there has been known that the speed deviation (.omega..sub.M *-.omega..sub.M) is caused to oscillate by Coulomb's friction between the motor and the torque transmission mechanism and the load machine. The torque component of Coulomb's friction is accumulated in the integrator of the speed control circuit 3, and then output as the torque instruction signal. As well known in the art, only when the motor is operated, Coulomb's friction is generated. When the motor is stopped, Coulomb's friction disappears so that the speed deviation is reduced, but the speed control circuit 3 outputs the toque component accumulated in the integrator as the torque instruction signal T.sub.M *. The torque control circuit 10 controls the torque of the motor 11 on the basis of the torque instruction signal T.sub.M *. This leads to a problem in that positional overshoot occurs.